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OTC 20340

Study on the Cost Effective Ocean Thermal Energy Conversion Power Plant Nagan Srinivasan, Specialist; Meena Sridhar, Deepwater Structures Inc., Houston, Texas, andMadhusuden Agrawal, ANSYS Inc., Houston, Texas

Copyright 2010, Offshore Technology Conference This paper was prepared for presentation at the 2010 Offshore Technology Conference held in Houston, Texas, USA, 3 –6May2010. This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not beenreviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, itsofficers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission toreproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright.

Abstract

Ocean Thermal Energy Conversion (OTEC) is a promising renewable energy resource from the ocean near equator that had been neglected for decades due to its enormous capital cost. The technologies that are developed for oil and gas industry fordeep water applications could be adopted into OTEC power plant and the capital cost could be reduced by fifty percent thanthe estimation using conventional OTEC technology. This paper addresses a new OTEC technology for the OTEC power plant. Design and workability of the individual components of the new OTEC engine is shown. The global architecture of the power plant is illustrated. The engineering analyses studied show that the new OTEC power plant is feasible to construct andthe cost analysis studied shows that the new OTEC power plant is viable for applications.

Introduction Clean renewable energy is a necessity of today that more and more people are aware of. Countries all over the world supportthis effort, considering that there is no carbon dioxide emission and no environmental impact. Ocean Thermal EnergyConversion (OTEC), as one of the renewable energy source, is very attractive because no fossil fuel needed and is availablewith unlimited large quantity. USA is now spending several billion dollars for various renewable energy source projects. Theoperating cost of most of the renewable energy is very low but the capital cost is enormous. Because of the environmental

nature, OTEC is very stable and reliable energy resource for tropical islands like Hawaii. The main challenge is to make theOTEC electric power generation cost-effective and comparable to the nuclear energy source.

OTEC

OTEC is a method for generating electricity utilizing the source of stored solar energy available in the ocean near the equator,where the surface water temperature is about 25 degree C or more due to heat from sunlight and at 3000 ft depth the watertemperature is about 4 degree C. A thermodynamic heat transfer method is used to convert the thermal energy to run the gasturbine. Ammonia gas is used as working fluid for the OTEC engine. Heat transfer is the major engineering task to beinvolved on the success of any efficient OTEC power plant. Conventional OTEC does the heat transfer effort on the top ofthe platform deck. The amount of water needed to cool the working fluid, ammonia, is enormous. There is a cold water pipein the conventional OTEC that is used to bring the cold water from 3000 ft down to the free surface. Similarly the gas heating process, where the surface warm water is used in the evaporator, faces the similar problem. Enormous amount of heavyequipment for handling large volumes of water on the surface of floating vessel has increased the vessel size and

consequently the capital cost was very high in the conventional OTEC. This technology is addressed in Reference 1.

New OTEC

This paper addresses an OTEC engine that uses new technologies like sub-sea condenser, sub-sea pump, submergedevaporator and independent floating-pipe buoy platform to transport working fluid from turbine outlet to the subseacondenser. Oil and gas field has offered innovative and cost effective solutions for deepwater applications. The technologiesdeveloped for deepwater are well proven and are working in water depth over 6,000 ft. OTEC water depth is only 1,500 ft,which is much smaller than the depth capabilities of today oil and gas field operations. The new OTEC design uses subseasolutions for the heat conduction problem. The condenser is located subsea area where the enormous cold water is availablewithout the need of storage. The condenser is directly exposed to the underwater environment with 4 degree C. Thecondenser is designed for 1,500 psi water depth pressure for its structural strength integrity. Subsea condenser is tested forflow simulation and heat conduction problem for workability by using modern ANSYS Computational Flow Dynamic (CFD)engineering software. The equipments and the piping are tested for 1,500 psi water pressure to work at 3,000 ft depth.

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An innovative subsea pump is also designed for the new OTEC. A supporting vessel is designed such that the evaporatorcould be placed submerged in the telescopic keel-tank. The keel-tank is a part of the supporting vessel thus it does not costextra for the support of the submerged evaporator of the new OTEC. The problems resulted with subsea condenser are solvedcost effectively in this paper with different innovative OTEC equipments. The paper aims at coming up with the first 100mega watt output power plant that could be operated in the State of Hawaii deep water. The new OTEC technologydrastically removed the massive cold water pipe from the conventional OTEC technology and reduces the cost significantly.This new OTEC technology is explained in Reference 2, emphasizing with various floating support vessel possibilities.

New OTEC-EngineFor the purpose of demonstration, a simplified OTEC cycle is used with the following assumptions. Sea bed is 3000 ft belowsea level; evaporator and expander are located at sea level; condenser and condensed ammonia pump are located at seabed;turbine operating condition is matched closely to the inlet and outlet conditions of an available General Electric (GE) turbine; power produced from the turbine is fixed at 20,000 kW. A pipe length equivalent of 3500 ft is used for turbine exhaust andcondensed ammonia is used for pump hydraulic calculations. An 84” ID for turbine exhaust and 24” ID for pump discharge are used as specified by GE for their turbine specification. However, actual piping design could be made of several smaller pipelines running in parallel. Those details are studied in the flow simulation problems in another section of this paper. Bothturbine exhaust line and condensed ammonia pump discharge line are also assumed to have heat losses . Both lines’ outlet and

inlet temperature differences are to be 0.5oC to approach “isothermal”. It is assumed that the condensed ammonia pumpdischarge control valve has 15 psi pressure drop. The new OTEC cycle is shown in Figure 1 and the Heat balance obtained inthe thermodynamics are show in Table 1. The heat energy balance is obtained as given in Table 2. The turbine inlet and outletconditions are obtained for 20 MW power output capacities.

An OTEC engine with subsea condenser located at 3000 ft seabed situation is compared with conventional OTEC, as shownin Table 3. The power consumption on the condensed ammonia pump is about 33.5% of power generated from the turbineexpander. In order to minimize this power loss, two pumps are used one underwater and a second one above water. Thesubmerged pump adequately transports the condensed working fluid to the evaporator at the top. Another axial pump is placed above water to suck the ammonia gas from the evaporator and send that to the turbine for the required input condition.This method reduces the total power consumption needed for the submerged pump. A new innovative displacement type pump is designed for this subsea application of the new OTEC power plant. The second pump located above water could beconventional pump available from standard vendors with large volumetric flow. In order to improve the performance of theturbine power output, a direct Sun heat chamber is used before the turbine inlet and the new thermodynamic cycle with thesechanges is named DeSI-OTEC as shown in Figure 2.

Description 101-

Expander-Inlet

102-

Expander- Outlet

104-

Condenser-Outlet

108-

Evaporator-Outlet

106-

Evaporator- Inlet

Vapor Fraction 1 0.99 0 1 0

Temperature [F] 84 56 39 84 43

Pressure [psig] 128 85 72 128 954

Molar Flow [lbmole/hr] 237,632 237,632 237,632 237,632 237,632

Mass Flow [lb/hr] 4,046,865 4,046,865 4,046,865 4,046,865 4,046,865

Heat Flow [Btu/hr] - 4,715,261,097 - 4,783,561,097 - 7,011,238,524 - 4,715,261,097 - 6,988,362,991

Table 1 HEAT AND MATERIAL BALANCE

Description Q-Expander Q-Condenser Q-Pump Q-Evaporator Q-P 100 Q-P 101Heat Flow [MMBtu/hr] 68 2,258 23 2,278 -46 -11

Power (kW) 19,998 661,270 6,698 667,007 -13,574 -3,125

Table 2 ENERGY STREAMS

Inlet Conditions Sub-Sea Condenser OTEC Conventional OTECMass flow (kg/hr) 1,836,000 1,835,646

Pressure (barG) 9.848 9

Temperature (oC) 29 29

Inlet Volume (Am3/hr) 251,993 247,626

Discharge ConditionsPressure (barG) 6.874 6

Temperature (oC) 13.56 14.2

Adiabatic/Polytrophic Efficiency (%) 83.225 / 82.926 84%

Outlet Volume (Am3/hr) 344,503 338,153

Power generated (kW) 20,020 20,000

Table 3 TURBINE INLET AND OUTLET CONDITIONS

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Figure 1: Simplified OTEC Engine withSubsea Condenser

Figure 2: A New OTEC Engine withSubsea Condenser

Subsea Pump

A dedicated subsea pump is designed for the new OTEC engine. A 3 dimensional view, a cross section at discharge strokeand a detail drawing with labels of parts of the subsea pump are shown in Figure 3 (a, b & c) as a group. The subsea pump isdesigned innovatively with two chambers, one with large size ammonia tank for transporting the condensed working fluidfrom the subsea condenser to the evaporator and another with small size water tank for driving the piston hydraulically. Theammonia chemical tank is designed larger in size with 4.5 ft diameter bore for more volumetric flow and the water tank isdesigned smaller with 2.5 ft diameter bore for smaller volumetric displacement. The piston in the chemical chamber(ammonia) and the piston in the water chamber are welded at both ends to a hollow cylinder that rides over a circular solidrod inside. The solid rod is welded on both ends to the two chambers’ edge walls. The chamber end walls are designed

convex outside to take external water pressure load of 1500 psi. A stronger spring is placed in the chemical side of thechamber and a weaker spring is placed in the water side of the chamber for operational stability. The piston in the waterchamber is driven by the high pressure water pump placed on top of the platform deck and is connected to the water chamber by an inlet pipe as shown in Figure 3. The maximum piston stroke distance is 12 inches and equal for both the chambers.

The water is pumped inside the water chamber with fluctuating pressure to drive the subsea pump. When water is suppliedwith pressure into the water chamber, it moves the piston from left to right side. Correspondingly the piston on the chemicalside is moved in the same direction. The check valves provided radially on the surface of the larger chamber open-up and thecondensed ammonia is sucked into the discharge side (right side) of the larger chamber. During the suction stroke, the springon the larger ammonia chamber is compressed to maximum. When the pressure on the water inlet pipe is reduced to normal,then the piston is designed to slide back from right to left by the reaction of the larger spring force.

Figure 3 (a & b): Subsea Displacement Pump - shown in Three Different 3D and 2D Views

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Figure 3 (c): Subsea Displacement Pump - shown in Three Different 3D and 2D Views

Several required check valves are used for the operation of appropriate pumping mechanism. The chemical chamber isdesigned for 1500 psi external hydraulic pressure at 3000 ft water depth with zero internal pressure. The water chamber isfilled with water such that the internal and the external pressures are equal. A spring loaded check valve is used on thedriving side of the water chamber for applicability. When the internal pressure exceeds the external pressure, then the valvecloses and the piston drives. As the piston in the ammonia chemical chamber moves back and forth, the condensed ammoniais being sucked from the subsea condenser and moved to the other side with the help of the check valves provided on the piston surface. The discharge stroke is shown in the 3D cross-section view and the suction stroke is shown in the 2D-detaildrawing. The mass flow of the working fluid from turbine output is 4046885 lb/hr. The gallons per minute (gpm) of theliquid ammonia fluid required to be pumped up by the subsea pump is 11852.52. The specific gravity of liquid ammoniamultiplied by the gpm volume flow rate and height in feet, divided by 3960 would give the Horsepower required for theammonia liquid to be pumped. A minimum of 4.6 MW power is required for the operation of this pump. In order to reduce

the water volumetric flow, inner bore diameter of the water chamber is reduced. The pump could be designed for a rate of 14strokes per minute with 2200 psi water pressure at the water inlet at the top of the deck. Thus the control of the operation ofthe subsea pump is feasible from the deck. The water chamber is designed for 2300 psi additional internal pressure.

Figure 4: Subsea Condenser with Propellers

Figure 5: Independent Vertical Float Buoy

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Subsea Condenser

A dedicated sub sea condenser is designed for the new OTEC engine. The condenser has inlet and outlet tanks. For efficientcondensation of ammonia by cooling, a shell type condenser is used underwater and is shown in the Figure 4. Thinrectangular tubes are bent and placed between the inlet and the outlet tanks. The rectangular tube is designed with a narrowgap of 3 inch width. The tube is internally supported with vertical plates to withstand hydrostatic outside water pressure of1500 psi. The spacing between the tube’s bend is designed adequate for the surrounding sea water to flow naturally in between without resistances. Several electric propellers (Figure 4) are mounted on the other side of the condenser such that astream of cold water flow is induced from one side to the other of the condenser through the designed spacing between thetube bends. These sub sea propellers that will induce the flow are operated hydraulically from the platform deck top throughflexible hoses.

The structural integrity of the subsea condenser is verified for 3000 ft water depth external pressure application. For efficientheat conduction, aluminum is used as the material for the subsea condenser tubes. The mass flow and the heat conduction problems are solved with the ANSYS Computational Flow Dynamic (CFD) program. The results of the analysis areillustrated in the following section of this paper. Similar design concept is applied for the submerged evaporator situatedcloser to the free surface. The CFD and the heat conduction studies are carried out for the evaporator.

Simulation of Ammonia Flow and Heat Conduction

This section of the paper presents the results of CFD and heat transfer analyses for the subsea condenser and submergedevaporator using ammonia as the working fluid and the surrounding sea water as the cooling or heating media fluid. FirstCFD modeling was performed to study the flow behavior of the ammonia gas from the turbine to the top of the vertical pipe-

buoy though smaller size pipes with the use of a manifold at the turbine outlet and at the top of the vertical pipe buoy.ANSYS FLUENT software was used for this CFD simulation. The results were used to obtain the optimum geometry of theinlet pipes. Ammonia vapor exits from the turbine through a pipe of 7ft diameter outlet at about 25 m/s. A manifold isrequired to split the ammonia flow into several smaller pipes which can be fed into the vertical pipe-buoy [Figure 5] at thetop through a similar size manifold. Another manifold is designed at the bottom of the vertical pipe-buoy to distribute theammonia gas into several smaller subsea condensers located on the seabed. The purpose of this second manifold is to reducethe velocity of the hot gas ammonia which is supplied to the condenser inlet for effective cooling. Both of these manifoldswere modeled and geometries of these manifolds are shown in Figure 6 and in Figure 8.

A diffuser on the turbine side is used which expands from a 7ft diameter to a 12ft diameter outlet which is then branched outto six 3ft diameter pipes. The diffuser at bottom of the vertical pipe-buoy expands from the 8ft diameter to 12ft diameter andthen to 20ft diameter outlet, where nineteen 3ft pipes are branched out to the several sub sea condensers. Isothermal CFDsimulation was performed for these manifolds design to obtain flow pattern and pressure field. Figure 7 shows contours of

velocity magnitude for the turbine side diffuser. Ammonia flow was uniformly divided into all the outlet pipes and theaveraged velocity in each pipe was about 25 m/s. Figure 9 shows contours of velocity magnitude for the bottom diffuserwhere the average velocity in each outlet pipe is about 7.5 m/s. Pressure drop for these two manifolds design are insignificant(~0.018 – 0.04 bar). A simplified analysis of the heat transfer problem was performed with straight tubes for the subseacondenser to study its efficiency with the assumption of 4oC constant outside temperature condition. It was discovered thatstraight tubes will not provide enough residence time and surface area for heat transfer to ammonia vapor. To increase thesurface area as well as residence time, U-bends were introduced in the design of these heat transfer tubes. 2D simulation ofthe cross-section of single tube was performed to optimize the inside gap of the flow and the spacing of U-bends in the tube.The objective was to obtain a target temperature around 4 to 5 oC at the outlet of the sub sea condenser with surrounding coldsea water.

A sensitivity analysis was performed to determine effect of the inlet gas velocity on performance of the heat transfer on thecondenser tubes. Two different gaps, 3 and 4 inches, for the flow inside the tube and three different u-bend spacing for the

tube were used for this study. The effect of three configurations of condenser tube on the temperature of the outlet workingfluid is shown in Figure 10 as contour plots. By introducing more u-bends on each tube, the overall flow-path for theammonia vapor and the total contact area of tube increases. This leads to more heat transfer and hence lowered thetemperature at outlet. Figure 11 shows plot of outlet temperature for these different configurations of condenser tubes againstthe inlet velocity. It clearly shows that with higher inlet velocity, outlet temperature is higher as less heat transfer occurred.Also as spacing between u-bends is decreased, while keeping the total height same, there are more u-bends and more surfacearea for heat transfer and hence lower temperature at outlet. 3D simulation of one typical condenser tube was performed. Thesurrounding cold sea water stream perpendicular to the direction of the ammonia working fluid was included in this CFD heatconduction model. In previous 2D simulation, for simplicity, a constant 4 oC temperature was assumed at the outer surface ofthe condenser tube. By including the surrounding water flow in the 3D model, accurate calculation of convective heat transferfrom water to condenser tube is achieved. Figure 12 shows the geometry of the 3D model for this simulation.

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Figure 6: Turbine Side Manifold Figure 7: CFD Simulation of Turbine side

Manifold - Contours of Velocity Magnitude

Figure 8: The manifold at Bottom ofVertical Pipe-Buoy

Figure 9: Vertical Pipe-Buoy Manifold – Contours of Velocity Magnitude

The ammonia gas is flowing inside the tube from top to down while water flow is outside of the tube from left to right.Figures 13 and 14 show temperature contours on middle vertical planes. Temperature of ammonia vapor decreases as it flowsdown. Figure 15 shows the temperature on the outer surface of the tube, which ranges from 4 oC to 5oC. Assumption ofconstant 4oC in previous 2D simulations was on lower side which has slightly over predicted the heat transfer. However,temperature at the outlet of the tube is sufficiently lower to ensure complete condensation of ammonia vapor.

7ft

12ft

7ft

12ft

20ft

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Similar study was performed to obtain the behavior of the evaporator with the same tube arrangement. In this case ammonialiquid at 4oC is entering from the bottom of the tube and the surrounding water flow is at 24oC, as shown in Figure 16. Figure17-18 shows temperature field for this evaporation simulation. Temperature is increasing from bottom to top as the heattransfer occurs from hot water. As shown in Figure 19, the temperature on the outer surface of evaporator tube is rangingfrom 19oC to 24oC. Temperature at the outlet of the tubes is sufficiently high to evaporate all liquid. The effect of water flowrate was also investigated in the above 3D simulation of the condenser and the evaporator. Water flow rate was varied from0.1 m/s to 2 m/s and ammonia temperature at the exit of plates was compared. Temperature change for water flow was also

compared. Figure 20, shows x-y plot of exit temperature vs. water flow rates for the condenser system. In the same Figure 20,similar plot for the evaporator system is shown. It is observed from these plots that water flow rate of around 1 m/s should bedesired to provide required heat transfer for the condenser and the evaporator heat transfer problems. A second evoprator is placed on the platform top which uses further heated warm water by the concentric solar panels as shown in Figure 2 before.The 3D-simulation for heating of ammonia gas from 22 to 30

oC temperature is performed. It used 30 oC water temperature at .1m/s to 0.5

m/s range and ammonia gas is 22 oC with 10 m/s at the inlet. The outlet temperature of ammonia gas was about 28.132

oC to 29.05 oC.

Figure 10: Contours of Static Temperature

Figure 11: - Study of Outlet temperatureVs. Inlet Velocity

Figure 12: 3D of Single Channel for Condenser Figure 13: Temperature Contours on Middle c/s Plane

Water

Flow

Ammonia flow

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Figure 18: Temperature Contours on Vertical Plane Figure 19: Temperature on Outer Surface of Plates

Effect of Sea Water Velocity Through the Heat Exchangers:

Top Two Curves for Evaporator & Bottom for Condenser

2

4

6

8

10

12

14

16

18

20

22

24

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Water Velocity (m/s)

T e m p e r a t u r e C a t

O u t l e t

NH3 Temp. at out let W at er Temp at Ex it NH3 Temp. W at er Temp.

Figure 20: Water Velocity vs. Ammonia Temperatureand Water Temperature at Exit

OTEC Power Plant Support

Truss-Pontoon Semi-Submersibles DTSP)

Main column

Deck

Truss pontoon

Knee brace

Hinge

Telescopic Column

Keel tank

C- frame

Figure 21: OTEC Supporting

Vessel Concept

Supporting VesselSix GE turbines will be used, each with 20 MW (see Table 3). Out of these, five turbines will be used to bring the total outputof 100 MW and the other one will be used for the subsea pump and other power supply requirement on the platform deck. Asupporting vessel is designed for the new OTEC engine. The vessel carries some ammonia storage, the turbines, pumps, solarheat chamber, living quarters with heli-deck on top. The vessel is designed for harsh environment application of the ocean. Adetailed technical merit of this vessel is given in Reference 3. It is a self-installing mobile offshore floating platform and isshown in Figure 21. The platform is sized for a topside deck payload of 2500 metric tones (5,500 kilo pounds) for supportingthe new OTEC system. The platform consists of four 25 ft square columns placed at a distance of 105 ft center to center with75 ft submerged depth and with 50 ft of free board. The platform keel tank is 130 ft by 130 ft square with 30 ft by 30 ft openat the middle and is 10 ft deep. It has the telescopic feature to 70 ft down further. Thus the total draft of the semi-submersibleduring operation is 155 ft. The estimated hull weight without the deck pay load is 11,700 kilo pounds.

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The naval architecture stability engineering and motion behavior are studied for this vessel and is shown in Table 5. Thevessel is highly stable and behaves well for the harsh environment of the ocean. The vessel heave natural period is 27 s. Theheave Response Amplitude Operator (RAO) of the vessel is shown in Table 5 as an insert within the table. The telescopickeel-tank accommodates the submerged evaporator and is exposed to the open sea at 140 ft below free surface. Propellerswould be mounted to simulate the water flow inside the evaporator, similar to the subsea condenser design. Six independentvertical floating-pipe buoys are used to transport the ammonia from each turbine outlet to the subsea condensers as shown before in Figure 5.

Independent Floating Vertical Pipe-Buoy The vertical floating buoy is an 8 ft outer diameter pipe with 1 inch wall thickness for about 750 ft and rest 750 ft as 2 inchwall thickness. It is designed for external water pressure with internal stiffeners. The pipe buoy has a 12 ft diffuser at the topand 20 ft diffuser at the bottom. The top diffuser receives six 3 ft diameter pipes from the turbine outlet. The bottom diffusersupplies the hot gas to the subsea condenser through nineteen 3 ft diameter pipes. Thus the inlet velocity of the flow to thecondenser is controlled below 10 m/s. The independent vertical pipe-buoy is moored at the bottom to the seabed and freefloats vertically up. The center of gravity of the pipe-buoy is set below the center of buoyancy for stability requirement likeSAPR vessel. Because of the 3000 ft deep pipe buoy, the vertical natural period is over 60 sec, which is above the waveexciting periods of the ocean. The diffuser at the bottom provides additional added mass. A flex joint or a universalmechanical joint could be used at the top to eliminate bending of the pipes. Special large diameter flexible hoses also could be used for this purpose. The vortex shedding behind the pipe is small because of the high Reynolds number in the order of10e5 to 10e6. The flow around the pipe is predominantly turbulent. The vertical float buoy pipe of 8 ft diameter could bedesigned for tension with additional ballast load at the bottom and additional buoyancy chamber near free surface (not shownin the Figure 5 illustration).

The independent floating buoy is a feasible design concept from the structural, naval architectural and hydrodynamic point ofview. This concept eliminates the need for supporting the six 8ft outlet pipes all the way to 3000ft by integrating with the platform unit. The platform is located at the middle of the six vertical floating buoys. The global architectural view of thecentral OTEC supporting vessel with the six independent floating buoys is shown in Figure 22. The independent vertical pipe buoy could act as the mooring for the central vessel which carries the major equipment. The independent vertical floating- pipe buoy will be locally manufactured and installed on the site. It would be installed by ballasting at the keel tank to make itupright. The pipe also could be installed vertically with the help of the platform through the moon pool by making threadedconnections. In that case, a derrick is placed on top of the platform deck to lower the pipe buoy by section by section.

The entire operation could be self installed with platform cranes. The independent floating buoy itself could be used for tosupport the turbine and the evaporator at the top with help of a smaller platform. The cost of such power plant for 20MW

output is very small compared to the plan of using a larger support vessel for 100 MW unit power output. It is wise to use thissingle vertical pipe buoy for the purpose of the pilot plant before the large 100 MW power plant is being built.

Figure 22: Global Architecture of the 100 MW OTEC:With its Independent Vertical-Pipe Floats Figure 23 DeSI Desalination Plant

Concept Diagram

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Sub sea CablesOTEC power plant needs subsea cables for transporting the electrical power to shore for feeding into the high voltagenetwork. Several large cable suppliers are available in the world market (Examples are ABB, Prysmian, Nexus and Draka). Afew of the manufacturers have over 25 years of subsea cable experiences. Power cable umbilical is used in offshoredeepwater platforms to deliver electrical power to subsea. Duco manufactures different materials for umbilical fromAluminum, Copper, and light weight nano Umbilical with different sizes from 118 mm to 77 mm. Their dry weight variesfrom 23.3 kg/m to 11.6 kg/m. The submerged weight varies from 12.9 kg/m to 7.3 kg/m. The operating voltage is 11 kV and

their electric performance for 10 miles is 3.0 MW power transmissions. Research is on going in Rice university of Houston,Texas, on the reduced size and weight of the power cable umbilicals with higher capacity. The research objective is to design, build and test an engineering prototype of a working ultrahigh conductivity “wire” that could in later stages be incorporated

into an umbilical exceeding 100 miles in length and called upon to deliver up to 10 MW at up to 36 kV with an operatingenvelop ranging from -1 to 121ºC (~ 30 – 250ºF) and pressure from 0 to 4500 psi. This would benefit OTEC power plant.

Desalination a by-Product

Desalination could be a by-product of OTEC power plant or it could be independent too. Subsea condenser and subsea pumpare used as similar to DeSI-OTEC power plant. The ammonia used as working fluid is cooled underwater at 3000 ft or lessdepth (5 to 12 degree C) and pumped up to the surface in a closed cycle with the help of a subsea pump. The warm wateravailable near free surface (25 to 28 degree C) is flashed into a vacum chamber. The water is steamed at low temperature andthen passes through the evoprator placed on the platform. The water vapour is condensed in the evoprator chamber where thecooled ammonia is used as a coolent. The ammonia is recyled back to the underwater condenser. This technology eliminatedthe need of large cold water pipe and uses ammonia as the working fluid. Since ammonia is condensed, the pipe sizes aresmall compared to the cold water pipe. The size of the water condender described above is very small. Efficiency ofAmmonia for heat transfer is much faster than the cold water to condense the water vapour to produce potable water. Theover all cost is reduced signifincantly and the effeciency is increased with this DeSI Desalination Plant. Figure 23schematically illustrates the desalination plant with subsea condenser and subsea pump concept.

Cost Analysis

Capital cost is the major objective of the OTEC for its success. A cost analysis is needed to illustrate the advantage of thealternate OTEC over the conventional solution. The platform and the subcomponents of the OTEC engine as designed in this paper have shown significant cost reduction from 40 to 60% compared to the conventional OTEC. Table 6 below shows theestimated cost of the 100 MW OTEC power plant with its subsea equipment and the supporting vessel. The total estimate ofthe capital cost is $400 million USD. Table 7 shows the cost analysis for the OTEC project from commercializationfeasibility point of view. It uses 95% reliability factor with 30 year life span for the 100 MW. The interest payments arerolled into the loan until the start of the production. The electricity cost is assumed as $0.14 per KWh which is cost effective

and could be further reduced depending on the type of loan. Significant profit is seen in the Table 7. This shows OTEC withsub sea solution is attractive for real world applications.

Conclusion Clean energy and clean water are the fundamental needs. Many islands and main lands near equator could benefit if these twocould be produced cost effectively. This paper tried to achieve this goal with the knowledge that is gained from the deepwateroil and gas. The paper addressed an alternate solution for the OTEC with a subsea condenser. The thermodynamics, turbine,heat conduction, flow problems of working fluid and the supporting vessels are studied for the feasibility of the alternatesolution of the new OTEC. It is concluded that the new OTEC is a promising renewable energy power source for a significant part of the world near equator. Further improvements to this new OTEC could be possible with the oil and gas deepwatertechnology. The new OTEC is definitely a good renewable energy source to be considered by the industry seriously. A jointindustry effort is needed with support of the Federal Government for this new OTEC as a source of pollution free, renewable,alternate source of energy for many countries of the world.

Acknowledgement

The authors thank Dr. Bala N. Aiyer for his help in correcting the paper. The new OTEC technology and its components are patent protected and are also proprietary technologies of the second author’s company. The authors thank Dr. Robert Cohenfor his encouragement to work on the OTEC topic using oil and gas deepwater technology.

References 1. Robert Cohen, Consultant, An Overview of Ocean Thermal Energy Technology, Potential Market Applications, and Technical

Challenges, OTC20176, 2009.2. Srinivasan, Nagan, et.al. A New Improved Ocean Thermal Energy Conversion System with Suitable Floating Vessel Design, OMAE2009-

80092, May 31 – June 5, 2009, Honolulu, Hawaii, USA. 3. Srinivasan, Nagan, et.al. Innovative Harsh Environment Dry-Tree Support Semi-Submersible for Ultra-deepwater Applications,

OMAE2009-80085, May 31 – June 5, 2009, Honolulu, Hawaii, USA.

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Table 5: Supporting Vessel Stability & Motion Studies

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